Essential Grid Lightbox Problem

Greetings,

I am using the Essential Grid for displaying some images on my pages. It works wonderful on every browser except for Firefox.

In Firefox, when an image is clicked and goes into “lightbox” mode, there is a black box that appears in the upper right corner and goes down the right side of the image. Also, the “X” circle for closing the lightbox turns into a blank white square.

Please see the attached images for a visual of the problem.

Any ideas as to what may be causing this?

Thanks!